Method for oxidizing hydrocarbons

ABSTRACT

A process for oxidizing polyalkyl monoaromatic hydrocarbons to the corresponding polycarboxylic monoaromatic acid by contacting them with an aqueous solution of ammonium dichromate at elevated temperatures. The product of the reaction is the ammonium salt of the corresponding polycarboxylic acid, which, upon acidification, yields the free organic polycarboxylic acid.

United States Patent Benham et al. *Jan. 7, 1975 METHOD FOR OXIDIZING[58] Field of Search 260/524 M, 531 R; 23/56 HYDROCARBONS [75]Inventors: Alvin L. Benham, Clear Lake, Tex.; [561 References CitedDennis E. Drayer, Littleton, Colo.; UNITED STATES PATENTS Harold D.McBride, Lincoln, Nebr. 3,560,559 2/1971 Benham et a] 260/524 [73]Asslgnee: g fi s Company Fmdlay Primary Examiner-Lorraine A. WeinbergerAssistant Examiner-Richard D. Kelly Notice: The portion of the term ofthis Attorney, Agent, or FirmSughrue, Rothwell, Mion,

patent subsequent to Feb. 2, 1988, Zinn & Macpeak has been disclaimed.22 Filed: Dec. 18, 1970 [57] A process for oxidizing polyalkylmonoaromatic hyi i PP NOJ 99,380 drocarbons to the correspondingpolycarboxylic Related Application Data monoarornatic acid by contactingthem with an aque- [63] C ti v t f S r NO 408 884 N 4 ous solution ofammonium dichromate at elevated 52 gz wg g gg 2 temperatures. Theproduct of the reaction is the ammonium salt of the correspondingpolycarboxylic acid, [52] U S Cl 260/524 M 260,518 R 260,523 A which,upon acidification, yields the free organic poly- D u--.- a a 260/531 R,260/533 C, 260/558 R Int. Cl C07c 55/14, C07c 63/02 carboxylic acid.

18 Claims, 1 Drawing Figure PAIENTED JAN 7 S RECYCLED AMMONIUMDICHROMATE FEEDSTOCKW 1 OXIDATION CHROMIC oxmE REMOVAL ACIDIFY AMMONIUMSALT PRODUCT REGENERATION AMMONIA REMOVAL RECYCLED AMMONIUM- DICHROMATETO OXIDATION INVENTORS ALVIN L. BENHAM DENNIS E. DRAYER HAROLD D.MCBRIDE M, Ki/5141c, 014,80 Z114 WQ K ATTORNEYS METHOD FOR OXlDlZlNGHYDROCARBONS This application is a continuation-in-part of U.S.application Ser. No. 408,884, filed Nov. 4, 1964, now U.S. Pat. No.3,560,559.

The present invention relates to the oxidation of cyclic hydrocarbonsand partially oxidized cyclic hydrocarbons by ammonium chromate ordichromate. More particularly, the invention relates to the oxidation ofthese compounds by reaction with aqueous ammonium chromate ordichromate.

Prior oxidation processes utilizing a chromium (VI) oxidant areeconomically unfeasible for commercial application because they resultin the formation of large amounts of alkali metal halides or sulphateswhich are of low value. According to the present process, the productionof large amounts of low value byproducts is avoided and the reducedchromium compound, Cr O is regenerated to the dichromate while producingthe more economically attractive ammonium salts.

The following is a general description of the present process. Anunderstanding of the major steps of the process and their inter-relationmay be facilitated by reference to the accompanying flow diagram.

A feedstock containing cyclic hydrocarbons and/or partially oxidizedcyclic hydrocarbons is contacted with an aqueous solution of ammoniumdichromate at elevated temperatures to form various oxidation products.Depending upon the pH of the reaction media, the oxidation productformed will be either the ammonium salt of an organic carboxylic acid oran organic carboxylic acid amide.

The primary products formed by the present invention are carboxylicacids or carboxylic acid amides. At pH levels of about 7 or below, thepredominant product is the ammonium salt of an organic carboxylic acidwhich can be acidified to form the free acid. When the pH of thereaction media is above about 7, high yields of carboxylic acid amidesare formed.

During the oxidation reaction, the ammonium dichromate is reduced to CrO which precipitates from the reaction mixture and is removed byconventional means. After being removed from the reaction zone, the Cr Ois mixed with ammonium hydroxide and oxidized with oxygen to formammonium chromate which can be converted to ammonium dichromate. Theregeneration of Cr O is enhanced by conducting it in the presence ofcatalysts, such as copper sulphate or cobalt naphthenate. In a preferredembodiment, the Cr O is regenerated in the presence of a mixture ofcupric sulphate and sodium sulphate. The copper ions, whichdetrimentally effect the oxidation reactions, are precipitated from theregeneration mixture by removing ammonia from the mixture aftercompletion of the regeneration period. 7

Following the removal of the Cr O from the oxidation product, water andammonia are flashed from-the hot reaction mixture. The oxidation productis then recovered. When the product is the ammonium salt of an organiccarboxylic acid, it is reacted with a nonoxidizing strong acid to formthe organic free acid and an ammonium salt of a strong acid.

The regeneration of Cr O to form ammonium chromate is more fullydisclosed in application Ser. No. 373,879, filed June 9, 1964, now US.Pat. No. 3,393,972, and Ser. No. 402,958, filed Oct. 9, 1964,

now US. Pat. No. 3,369,86l, both assigned to the assigneeof thisapplication.

The present invention has utility in the oxidation of a wide variety ofhydrocarbons and partially oxygenated hydrocarbons. Examples of suitablefeedstock materials include the xylenes, mesitylene, durene,propylbenzene, cymene, toluic acid, toluol, tolualdehyde,2,6-dimethylnaphthalene, acenaphthene, acenaphthylene,l-methyl-4-isopropylnaphthalene, 1,4-dimethyl- 4-isopropylnaphthalene,1,3-dimethylanthracene, 2,7-

dimethylanthracene, 1,7-dimethylphenanthrene,1,6-diisopropylnaphthalene, 1,2,4- trimethylanthracene, 7-methyl- 1-ethylphenanthrene,

1-methyl-4-isopropylanthracene, 1,3,6,8-tetram ethylanthracene, I9,l0-diethylphenanthrene, cyclohexane, cyclopentane, cycloheptane,cyclohexanol, and cyclopentanol. Generally, any aromatic compoundsubstituted with a lower alkyl group can be used as a feedstock.Mixtures of hydrocarbons can also be oxidized by this procedure. Anexample of such a mixture is cycle oil, a product of the catalyticrefining of petroleum (Industrial and Engineering Chemistry, Vol. 38,pp. 136-( 1946) at page 137). Generally these products are a mixture ofalkylnaphthalene, anthracene, etc.

The oxidation of the feedstock is generally accomplished by mixing thealkylaromatic or cycloaliphatic compound with an aqueous ammoniumdichromate solution and heating. The reaction conditions may vary overfairly wide ranges. The oxidation temperatures which are utilized varywidely with the raw material. Thus, temperatures from about 200 to 325Care suitable temperatures for the preparation of the carboxylic acidamides, preferably carried out in the range of from to 325C. The pH ofthe oxidation reaction mixture can vary from about 1 to about 11, with apH below about 7 being preferable for the preparation of high yields oforganic carboxylic acid products. The ratio of dichromate equivalents tooxidizable carbon atoms which are useful in the process range from about0.5 to l to in excess of 3 to l. A ratio in the range of from about1.5:1 to 25:1 is preferred. High yields are obtained with relativelyshort reaction times on the order of about 15 minutes to 30 minutes. Thereaction may be conducted at ambient pressures.

The oxidation reaction mixture is in two phases and the known techniquesfor increasing miscibility and surface area are generally applicable tothe instant process. For example, surfactants can be added to themixture to reduce the surface tension between the two phases andstirring or bubbling can be used to increase interfacial area.

After oxidation is substantially completed, ammonia and steam areremoved from the reaction zone, preferably by flashing off thesecompounds. The precipitated chromic oxide is then removed, usually byfiltration, or by other means, such as decantation. The ammonium saltsof the organic acis in the reaction medium are then neutralized with anon-oxidizing strong acid, such as sulphuric acid, hydrochloric acid,hydrobromic acid, phosphoric acid, etc., to form the free organic acidwhich precipitates and is removed by filtration, decantation or similarwell-known methods.

In a preferred cyclic process, the Cr O is regenerated to form ammoniumdichromate. The chromic oxide is transferred to a regenerator, usually apressure resistant vessel or autoclave, in which it is reacted withoxygen and aqueous ammonia. Generally the chromic oxide is preferablymixed with aqueous ammonia, most of which is derived from the ammoniaremoval step which may be conducted as part of the process. The aqueousammonia solution may contain from about 4% to 80% by weight of ammonia,about being preferred, and the mole ratio of ammonia to chromic oxideshould be from about 2 to 160. Oxygen, in the form of pure oxygen,oxygen enriched air, air, and oxygennitrogen mixtures, is then added tothe autoclave containing this mixture to provide an'oxygen partialpressure of from about to in excess of 500 psi and preferably about 200psi. The reaction mixture is then heated for at least about 15 minutesat a temperature of from 140 to 225C and preferably about 180C. Thereaction is conducted in the presence of copper sulphate and sodiumsulphate. In this manner, about 90% of the spent Cr(l11), in the form ofCr O may be converted into Cr(Vl) in the form of ammonium chromate.

After the regeneration of chromic oxide is completed, ammonia is removedfrom the reaction vessel. Preferably, ammonia and steam are flashed fromthe reaction vessel. Additional heating at from about 150 to 250C can beused to flash off excess ammonia, thereby reducing the pH of thereaction mixture and precipitating copper ions as the insolublehydroxide. Ammonium dichromate is formed from ammonium chromate as theammonia is removed. The flashed ammonia is then recycled to the chromicoxide regeneration vessel.

Having described the invention in general and in terms of a preferredcyclical mode of operation, it is believed that the following detailedexamples of preferred procedures will assist towards a betterunderstanding of the process.

The invention will also be better understood by reference to the flowdiagram of the process which is set forth in the single FIGURE of theaccompanying drawmg.

EXAMPLE 1 A mixture of 154 ml of distilled water, 50.7 g (0.201 mole) ofreagent ammonium dichromate having an initial pH of 3.65 and 9.13 ml(0.074 mole) of p-xylene were placed in a 300 ml rocking autoclave andheated to 225C. The mixture was allowed to react for a period of 60minutes with continuous rocking of the autocla've.

The reaction products of the p-xylene oxidation were then filtered andacidified with hydrochloric acid to a pH of about 1 toprecipitate thearomatic acids. A 97.8% conversion of the p-xylene was obtained and atotal mixed acid yield of 86.5% was recovered. Upon analysis, it wasfound that about two-thirds of the mixed acids were terephthalic acidand most of the balance was terephthalamic acid. Base hydrolysis of themixed acids produced over 99% pure terephthalic acid and ammonia gas. I

The acidification ofthe oxidation products with hydrochloric acid alsoproduced ammonium chloride. This salt was recovered by filtration,purified by recrystallization, and then stored. In large scalecommercial process, the by-product ammonium chloride can be sold, thusrenderingthe process more attractive than prior methods in which sodiumchloride is produced.

EXAMPLE 2 Following the procedure of Example 1, a mixture of 154 ml ofwater. 50.7 g (0.201 mole) of ammonium dichromate and 15.62 gms (0.10mole)' 2,6-dimethylnaphthalene was added to a 300 ml rocking autoclave.The reaction was conducted during continuous rocking of the autoclave at225C for 1 hour. After flashing off excess ammonia, the oxidationreaction filtrate was acidified with hydrochloric acid to a pH ofapproximately 1 to precipitate naphthalene-2,6-dicarboxy1ic acid. Theyield was 38.8%.

Following the procedure of Example 1, a number of oxidations wereconducted using a variety of feedstock materials and varying some of thereaction conditions to determine the effect such changes may have onyield.

' The oxidation of o-xylene with aqueous (N-H Cr O was conductedfollowing the procedure of Example 1.

In each case, 0.201 mole of (NH Cr O and 0.123 mole of o-xy1ene wasreacted in 154 ml of H 0 for 60 minutes, with an initial pH of about3.5. The ammonium dichromate solution was free from residual cupricsulphate. The results of a series of such runs at varying temperaturesappear in Table 1.

From the data of Table 1, it is evident that the major product of theoxidation of o-xylene with ammonium dichromate is o-toluic acid.Relatively minor amounts of o-toluamide and phthalic acid are alsoproduced. The yield of o-toluic acid tends to increase at more elevatedtemperatures with the highest yield being obtained at 275C. The yieldcomputed from the results of the experiments reported in Table 1 andTable 2 is based on the mole percent of aromatic acid or amide producedper mole of o-xylene charged.

Another series of runs was conducted under exactly the same conditionsas were-employed in the reactions reported in Table 1, but the aqueousammonium dichromate solution contained 6.8 grams of residual cupricsulphate'catalyst. The results of this series of reactions are reportedin Table 2.

It is readily apparent from the data of Table 2 that the presence of asubstantial amount of residual copper ions in the oxidant solutionsharply reduces the yield of Table 4 Cone. Reaction p-Xylene Runp-Xylene H O NH OH Temp. Percent Percent yield No. ml ml ml (C)Conversion Acids p-Toluamide valuable aromatic acids. regardless ofvariation in temperature. This demonstrates the importance of the copperion removal step before the recirculation of the regenerated oxidantsolution. a

A number of oxidation reactions were also conducted utilizing p-xyleneas the alkyl aromatic compound. Still following the general procedure ofExample 1, 0.201 mole of (NH Cr O was reacted with varying amounts ofp-xylene in 154 ml of H 0. The reactions were conducted at a temperatureof 225C for 60 minutes. The initial pH of each reaction mixture wasadjusted to 3.65. The yields are computed as the mole percent of organicacid or amide produced per mole of hydrocarbon charged to the autoclave.Conversion is the mole percent of a given reaction component convertedto other products and includes any unrecovered portion of the component.The results of these runs are reported in Table 3.

The data contained in Table 3 indicates that substantial yields ofterephthalic acid may be obtained by the oxidation of p-xylene withammonium dichromate. It also shows that higher yields are obtained athigher mole ratios of (NH Cr O to p-xylene. A commensurate decrease inthe yield of secondary products is experienced as the (NH.,) Cr O top-xylene ratio is increased.

dium contained 6.8 g of cupric sulphate. The results of these reactionsare set forth in Table 4. The percent yield of acids below includesterephthalic plus terephthalamic acids.

The data of Table 4 indicate that higher yields of mixed terephthalicand terephthalamic acids are obtained at more elevated temperatures, themaximum yield being obtained at a reaction temperature of 225C. However,the results are relatively poor when compared with the yields obtainedfrom the reactions reported in Table 3 where an ammonium dichromateoxidant solution substantially free from copper ions is employed. Thisagain demonstrates the importance of the removal of copper ions from theoxidant solution.

A feedstock of 2,6-dimethylnaphthalene was also subjected to oxidationby aqueous ammonium dichro mate in another series of reactions conductedaccording to the general procedure of Example 1. The reaction wasconducted with 50.7 g (0.201 mole) (NH Cr O and varying amounts of2,6-dimethylnaphthalene in 154 ml of water. The initial pH of eachreaction media was 3.65. In addition to varying the amount of the alkylaromatic reactant, the reaction times and temperatures were also variedfrom one run to another. The different conditionscmployed in eachreaction and the results obtained appear in Table 5.

Table 3 (NH,),Cr,O Percent Yield Ru pXylene Percent p-XyleneTerephthalic Terephthalamic No (ml) Millimole Conversion Mole Ratio AcidAcid p-Toluamide Table 5 2,6-Dimethylnaphthalene Reaction ReactionNaphthalene-2,6- Run Percent Temperature Time Dicarboxylic Acid No.grams mole Conversion (C) Hours Percent Yield Another series ofreactionswas conducted to determine the effect of 1) reaction temperaturevariations and (2) the presence of copper ions, as cupric sulphate, onthe oxidation of p-xylene with ammonium dichromate. The same generalprocedure was employed as in the preceding oxidations. The amount ofreactants was the same in each run, 50.7 g (0.201 mole) of (NHJ Cr O and9.13 ml ofp-xylene. The reaction me- As shown in Table 5, substantialamounts of naphthalene2,6-dicarboxylic acid are produced according tothis process. The yield of the acid tends to be higher at more elevatedreaction temperatures as may be seen by comparing Run Nos. 5-1 and 5-2,5-4 and 5-5, 5-3 and 5-5. A comparison of the results of Run Nos. 5-2and 5-5 also indicates that a higher acid yield is obtained at highermole ratios of (NHJ Cr O to 2,6-dimethylnaphthalene. Some improvement inyield at longer reaction times is also evident from a comparison of RunNos. -2 and 5-7. 5

A further study of the effect of reaction time on the oxidation of2,6-dimethylnaphthalene was made in a series of reactions in which onlythe time was varied. In these runs, 50.7 g (0.201 mole) of (NH,,) Cr Oand 15.62 g (0.10 mole) of 2,6-dimethylnaphthalene in 154 ml H O werereacted at 215C. The results are set forth The data of Table 6demonstrate that the yield of the dicarboxylic acid increases withincrease in the reaction time up to about 6 hours. A correspondingincrease in 2,6-dicarboxylic acid selectivity is also experienced as thetime is increased progressively from 1 to 6 hours in Run Nos. 6-1through 6-4. However, between 6 and 8 hours reaction time, a decrease inyield and selectivityis noted, due perhaps to some degradation orrecombination of the products. In this context, selectivity refers tothe fraction of converted 2,6-dimethylnaphthalene'that is oxidized tonaphthalene 2,6- dicarboxylic acid.

A feedstock of m-xylenewas also oxidized with aqueous (NH Cr O In thesereactions, 50.7 g (0.201 mole) (NH Cr and 9.13 ml (0.074 mole) m-xylenein 1 54 ml H O were reacted for 60 minutes. The results appear in Table7.

Table 7 case, 50.7 g (0.201 mole) (NH Cr O was reacted with thefeedstock in 154 ml H O for 60 minutes. The results of these reactionsare set forth in Table 8. In this data, weight percent yield is weightof product/weight of starting material X 100.

Table 8 Org. Temper- Con- Run Start. ature version No. Material (C)Products Percent mole yield mixed durene acids including mellitic acid8-2 0.050 225 11.78 wt. 87.2

mole yield mixed durene acids including Py mellitic acid 8-3 15.5 ml 21512.5 wt.

, LCCO yield mixed extract acids 8-4 0.1 mole 215 18.0% adipic cycloacidhexanol As mentioned before, when the pH of the reaction media is inexcess of about 7, substantially increased yields of carboxylic acidamides can be produced. This embodiment of our invention will be betterunderstood by resorting to the following examples and tables.

EXAMPLE 3 300 ml rocking bomb was charged with 9.13 ml pxylene, 50.7 gammonium dichromate, 100 ml concentrated ammonium hydroxide (30% NH and54 ml distilled water. The pH of the aqueous phase before reaction was10.42. The reaction mixture was then heated at 225C for 60 minutes,after which the pH of the aqueous phase was 10.75. The aqueous effluentremaining after the completion of the oxidation reaction Reactionm-Xylene lsophthalic isophthalamic Run Temp. Percent m-Toluamide AcidAcid No. (C) Conversion Percent Yield Percent Yield Percent Yield Thedata in Table 7 show that a substantial yield of isophthalic acid isobtained by the oxidation of mxylene according to this process. Asignificant yield of isophthalamic acid is also produced. The variationin temperature from 215C to 235C does not produce a .great increase indiacid yield, but a large yield of isophthalamic' acid was secured at235C.

Mixed xylenes m-, 25% o-'and 25% p-xylene) were also subjected toammonium dichromate oxidation and substantial amounts of mixed acidswere produced.

A number of other feedstocks were oxidized according to the presentprocess with good results. In each addition of ammonia or sulphuric acidfrom runto run.

Otherwise, the same amounts of reactants and the same reaction conditionwas employed. The results appear in Table 9. The results of Example 3are also included in the Table for purposes of comparison.

Table 9 p-Toluamide- Tcrephthalamic Terephthalic pXy1ene Run lnitialFinal Percent Acid Acid Percent No. pH pH Yield Percent Yield PercentYield Conversion Table 9 p-Toluamide Terephthalamic Terephthalicp-Xylene Run lnitial Final Percent Acid Acid Percent No. pH pH YieldPercent Yield Percent Yield Conversion From the data in Table 9, it isevident that a sharply increased yield of terephthalamic acid andp-toluamide is obtained at pH levels above about 7. At lower pH levels,Run Nos. 9-1 through 9-4, aromatic acid production is favored.

1n the present data, yield is the mole percent of product per mole ofalkyl aromatic charged. Conversion is the percent of a given reactioncomponent converted to other products including any unaccounted forportion of the reaction component Reactions were also conductedaccording to the procedure of Example 3 using o-xylene as the feedstock.In each case, 9.32 ml of o-xylene was mixed with 50.7 g of ammoniumdichromate in 154 ml of water. The initial pH of the reaction mixturewas adjusted to the desired level by the addition of 10% H 50 or 58% NHOH to the reaction mixture. The reaction was carried out at 225C for 60minutes. The results of the runs appear in Table 10.

Table 10 oToluic Phthalic Run Initial Final Acid o-Toluamide Acid No pHpH Percent Percent Percent The data of Table 10 clearly show that morethan twice as much o-toluamide is produced by reaction at a pH above 7,than is formed at lower relatively acidic pH levels.

EXAMPLE 4 EXAMPLE 5 Example 4 was repeated, but the initial pH of thereaction mixture was adjusted to 6.9. A yield of 6.6% of mixed amideswas obtained.

EXAMPLE 6 Example 4 was repeated, but the initial pH of the reactionmixture was adjusted to 9.8. A yield of 7.9% of mixed amides wasobtained.

The results of Examples 4, 5 and 6 also demonstrate that conducting theoxidation of alkylaromatic compounds at a pH about 7 or higher resultsin a substantial increase in amide yield over that obtained by reactingat relatively acid pH levels.

lt will be understood that various changes in the de tails of theprocess may be made by those skilled in the art without departing fromthe spirit of our invention. It is our invention, therefore, to belimited only as indicated by the scope of the following claims.

What is claimed is:

l. A method for oxidizing a polyalkyl monoaromatic hydrocarbon feedstockto the corresponding polycarboxylic monoaromatic acid comprising:

a. reacting the feedstock with aqueous ammonium dichromate in anequivalent ratio of from about 0.511 to about 3:1 of said ammoniumdichromate to oxidizable carbon atoms of said polyalkyl monoaromatichydrocarbon and at a temperature of from C to 325C to produce chromicoxide and the ammonium salt of said polycarboxylic monoaromatic acid,

. reacting the chromic oxide with oxygen in aqueous ammonia to produceaqueous ammonium chromate, i c. removing ammonia from said aqueousammonium chromate to form aqueous ammonium dichromate, recycling theaqueous ammonium dichromate for the oxidation of additional feedstock,and d. acidifying the ammonium salt of said polycarboxylic monoaromaticacid to preciptate the acid therefrom.

2. The method of claim 1 wherein said feedstock is a polymethylsubstituted monoaromatic hydrocarbon.

3. The method of claim 2 wherein said feedstock comprises a memberselected from the group consisting of mesitylene and durene.

4. The method of claim 3 wherein said feedstock comprises mesitylene andthe corresponding polycarboxylic monoaromatic acid is trimesic acid.

5. The method of claim 3 wherein said feedstock comprises durene and thecorresponding polycarboxylic monoaromatic acid is pyromellitic acid.

6. The method of claim 1 wherein said chromic oxide is separated fromthe ammonium salt of the said polycarboxylic monoaromatic acid prior toreaction with oxygen, and further wherein said ammonium salt of saidpolycarboxylic monoaromatic acid is precipitated by acidifying theammonia free aqueous phase remaining after the reaction of the feedstockwith the ammonium dichromate.

7. The method of claim 1 wherein said polyalky monoaromatic hydrocarbonfeedstock comprises a member selected from the group consisting ofmesitylene which is oxidized to trimesic acid and durene which isoxidized to pyromellitic acid wherein said feedstock is reacted withsaid aqueous ammonium dichromate to produce an aqueous phase in which isdissolved the ammonium salt of the product acid of the feedstock and toproduce a chromic acid precipitate, and further wherein ammonia isremoved from said aqueous ammonium chromate by heating said aqueousammonium chromate to form aqueous ammonium dichromate and ammonia.

8. The method of claim 1 further comprising recovering ammonia generatedduring the process of recycling it as aqueous ammonia for reaction withsaid chromic oxide and oxygen to produce aqueous ammonium chromate.

phate.

9. The method of claim I wherein said chromic oxide, oxygen and aqueousammonia are reacted in the presence of a cupric sulphate catalyst andsodium sulphate.

10. A method for oxidizing a polymethyl monoaromatic hydrocarbonfeedstock to the corresponding polycarboxylic monoaromatic acidcomprising:

a. reacting the feedstock with aqueous ammonium dichromate in anequivalent ratio of from about 0.511 to about 3:1 of said ammoniumdichromate to oxidizable carbon atoms of said polymethyl monoaromatichydrocarbon and at a temperature of from 160C to 325C to produce achromic oxide precipitate and an aqueous phase containing the ammoniumsalt of said monoaromatic acid,

b. separating said chromic oxide and said aqueous phase,

0. catalytically reacting the chromic oxide with oxygen in aqueousammonia to produce aqueous ammonium chromate,

d..removing ammonia from said aqueous ammonium chromate by heating saidaqueous ammonium chromate to form aqueous ammonium dichromate,

e. recycling the aqueous ammonium dichromate for the oxidation ofadditional feedstock, and

f. acidifying the'aqueous phase containing the ammonium salt of saidmonoaromatic acid to precipitate the acid therefrom.

11. The method of claim wherein said feedstock comprises mesitylene andthe corresponding polycarboxylic monoaromatic acid is trimesic acid.

12. The method of claim 10 wherein said feedstock comprises durene andthe corresponding polycarboxylic monoaromatic acid is pyromellitic acid.

13. The method of claim 10 further comprising recovering ammoniagenerated during the process and recycling it as aqueous ammonia forreaction with said chromic oxide and oxygento produce said aqueousammonium chromate.

14. The method of claim 10 wherein said chromic oxide, oxygen andaqueous ammonia are reacted in the presence of a cupric sulphatecatalyst and sodium sullS..The method for oxidizing a polymethylmonoaromatic hydrocarbon to produce the corresponding polycarboxylicmonoaromatic acid comprising:

mixing said hydrocarbon with aqueous ammonium dichromate, the equivalentratio of said ammonium dichromate to oxidizable carbon atoms in saidhydrocarbon being in the range of from 0.5 to 3, heating saidhydrocarbon and aqueous ammonium dichromate in an oxidation reactor at atemperature offrom 200C to 325C for at least 15 minutes to produce anaqueous phase containing the dissolved ammonium salt of said acid and toprecipitate chromic oxide, separating said chromic oxide and saidaqueous phase, mixing said chromic oxide with aqueous ammonia, the moleratio of said ammonia to said chromic oxide being in the range of from 2to 160, heating the mixture of chromic oxide and aqueous ammonia in anautoclave in the presence of a cupric sulphate catalyst and sodiumsulphate to a temperature in the range of from l40to 225C, under anoxygen partial pressure of from 20 to 500 :psi, to produce aqueousammonium chromate containing some dissolved cupric sulphate, heatingsaid aqueous ammonium chromate to form aqueous ammonium dichromate andammonia and to precipitate hydrated cupric oxide, recycling said aqueousammonium dichromate to said oxidation reactor for the oxidation ofadditional hydrocarbon, and I acidifying said aqueous phase containingthe dissolved ammonium salt of said acid to precipitate said acid. 16.the method of claim 15 wherein said hydrocarbon is mesitylene and saidacid is trimesic acid.. 17. The method of claim 15 wherein saidhydrocarbon is durene and said acid is pyromellitic acid.

18. The method of claim 15 further comprising recovering ammoniagenerated during the process and recycling it to said autoclave.

1. A METHOD FOR OXIDIZING A POLYALKY MONOARCONTIC HYDROCARBON FEEDSTOCKTO THE CORRESPONDING POLYCARBOXYLIC MONOAROMATIC ACID COMPRISING: A.REACTING THE FEEDSTOCK WITH AQUEOUS AMMONIUM DICHROMATE IN AN EQUIVALENTRATIO FO FROM ABOUT 0.5:1 TO ABOUT 3:1 OF SAID AMMONIUM DICHROMATE TOOXIDIZABLE CARBON ATOMS OF SAID POLYALKYL MONOAROMATIC HYDROCARBON ANDAT A TEMPERATURE OF FROM 160*C TO 325*C TO PRODUCE CHROMIC OXIDE AND THEAMMONIUM SALT OF SAID POLYCARBOXYLIC MONOAROMATIC ACID, B. REACTING THECHROMIC OXIDE WITH OXYGEN IN AQUEOUS AMMONIA TO PRODUCE AQUEOUS AMMONIUMCHROMATE, C. REMOVING AMMONIA FROM SAID AQUEOUS AMMONIUM CHROMATE TOFORM AQUEOUS AMMONIUM DICHROMATE, RECYCLING THE AQUEOUS AMMONIUMDICHROMATE FOR THE OXIDATION OF ADDITIONAL FEEDSTOCK, AND D. ACIDIFYINGTHE AMMONIUM SALT OF SAID POLYCARBOXYLIC MONOAROMATIC ACID TOPRECIPITATE THE ACID THEREFROM.
 2. The method of claim 1 wherein saidfeedstock is a polymethyl substituted monoaromatic hydrocarbon.
 3. Themethod of claim 2 wherein said feedstock comprises a member selectedfrom the group consisting of mesitylene and durene.
 4. The method ofclaim 3 wherein said feedstock comprises mesitylene and thecorresponding polycarboxylic monoaromatic acid is trimesic acid.
 5. Themethod of claim 3 wherein said feedstock comprises durene and thecorresponding polycarboxylic monoaromatic acid is pyromellitic acid. 6.The method of claim 1 wherein said chromic oxide is separated from theammonium salt of the said polycarboxylic monoaromatic acid prior toreaction with oxygen, and further wherein said ammonium salt of saidpolycarboxyLic monoaromatic acid is precipitated by acidifying theammonia free aqueous phase remaining after the reaction of the feedstockwith the ammonium dichromate.
 7. The method of claim 1 wherein saidpolyalky monoaromatic hydrocarbon feedstock comprises a member selectedfrom the group consisting of mesitylene which is oxidized to trimesicacid and durene which is oxidized to pyromellitic acid wherein saidfeedstock is reacted with said aqueous ammonium dichromate to produce anaqueous phase in which is dissolved the ammonium salt of the productacid of the feedstock and to produce a chromic acid precipitate, andfurther wherein ammonia is removed from said aqueous ammonium chromateby heating said aqueous ammonium chromate to form aqueous ammoniumdichromate and ammonia.
 8. The method of claim 1 further comprisingrecovering ammonia generated during the process of recycling it asaqueous ammonia for reaction with said chromic oxide and oxygen toproduce aqueous ammonium chromate.
 9. The method of claim 1 wherein saidchromic oxide, oxygen and aqueous ammonia are reacted in the presence ofa cupric sulphate catalyst and sodium sulphate.
 10. A method foroxidizing a polymethyl monoaromatic hydrocarbon feedstock to thecorresponding polycarboxylic monoaromatic acid comprising: a. reactingthe feedstock with aqueous ammonium dichromate in an equivalent ratio offrom about 0.5:1 to about 3:1 of said ammonium dichromate to oxidizablecarbon atoms of said polymethyl monoaromatic hydrocarbon and at atemperature of from 160*C to 325*C to produce a chromic oxideprecipitate and an aqueous phase containing the ammonium salt of saidmonoaromatic acid, b. separating said chromic oxide and said aqueousphase, c. catalytically reacting the chromic oxide with oxygen inaqueous ammonia to produce aqueous ammonium chromate, d. removingammonia from said aqueous ammonium chromate by heating said aqueousammonium chromate to form aqueous ammonium dichromate, e. recycling theaqueous ammonium dichromate for the oxidation of additional feedstock,and f. acidifying the aqueous phase containing the ammonium salt of saidmonoaromatic acid to precipitate the acid therefrom.
 11. The method ofclaim 10 wherein said feedstock comprises mesitylene and thecorresponding polycarboxylic monoaromatic acid is trimesic acid.
 12. Themethod of claim 10 wherein said feedstock comprises durene and thecorresponding polycarboxylic monoaromatic acid is pyromellitic acid. 13.The method of claim 10 further comprising recovering ammonia generatedduring the process and recycling it as aqueous ammonia for reaction withsaid chromic oxide and oxygen to produce said aqueous ammonium chromate.14. The method of claim 10 wherein said chromic oxide, oxygen andaqueous ammonia are reacted in the presence of a cupric sulphatecatalyst and sodium sulphate.
 15. The method for oxidizing a polymethylmonoaromatic hydrocarbon to produce the corresponding polycarboxylicmonoaromatic acid comprising: mixing said hydrocarbon with aqueousammonium dichromate, the equivalent ratio of said ammonium dichromate tooxidizable carbon atoms in said hydrocarbon being in the range of from0.5 to 3, heating said hydrocarbon and aqueous ammonium dichromate in anoxidation reactor at a temperature of from 200*C to 325*C for at least15 minutes to produce an aqueous phase containing the dissolved ammoniumsalt of said acid and to precipitate chromic oxide, separating saidchromic oxide and said aqueous phase, mixing said chromic oxide withaqueous ammonia, the mole ratio of said ammonia to said chromic oxidebeing in the range of from 2 to 160, heating the mixture of chromicoxide and aqueous ammonia in an autoclave in the presence of a cupricsulphate catalyst and sodium sulphate to a temperature in the range offrom 140*to 225*C, under an oxygen partial pressuRe of from 20 to 500psi, to produce aqueous ammonium chromate containing some dissolvedcupric sulphate, heating said aqueous ammonium chromate to form aqueousammonium dichromate and ammonia and to precipitate hydrated cupricoxide, recycling said aqueous ammonium dichromate to said oxidationreactor for the oxidation of additional hydrocarbon, and acidifying saidaqueous phase containing the dissolved ammonium salt of said acid toprecipitate said acid.
 16. the method of claim 15 wherein saidhydrocarbon is mesitylene and said acid is trimesic acid.
 17. The methodof claim 15 wherein said hydrocarbon is durene and said acid ispyromellitic acid.
 18. The method of claim 15 further comprisingrecovering ammonia generated during the process and recycling it to saidautoclave.